An injection system of “hot block” or “hot runner” type usually comprises:                a rigid moulding die having a front face partly delimiting the mould cavity, a rear face and at least one longitudinal passage passing through the die from its rear face to its front face,        a longitudinal injection nozzle, arranged coaxially inside said passage and comprising a longitudinal channel,        a manifold feeding the channel of the nozzle with thermoplastic material in the fluid state, located behind the rear face of the die and mechanically connected thereto, said manifold comprising a longitudinal feed channel leading towards the front in fluid relationship with the longitudinal channel of the nozzle.        
This type of injection system further comprises feed means capable of providing the manifold with material to be injected. For satisfactory injection of material into the mould cavity, the material must be held in the fluid state, this state being obtained when the material is brought to a determined limit temperature, higher than the temperature of ambient air. For this purpose the manifold, as is known per se, comprises means allowing the holding of its temperature, and hence the temperature of the material transiting through its dispensing channel, at a so-called “injection temperature” higher than the limit transition temperature to the fluid state of the material.
The material in the fluid state is placed in the dispensing channel of the manifold via feed means and enters into the transit passage of the injection nozzle. The injection nozzle is also provided with heating means which typically consist of an electric resistance wound around the nozzle. It is additionally necessary to ensure sealing of the nozzle both at its rear end, with respect to the manifold, and at its front end, with respect to the front face of the die. Existing means for ensuring a seal at the two ends of the nozzle do not give full satisfaction however.
FIGS. 1A to 1C illustrate three widespread modes for securing the injection nozzle. In these three systems, the seal at the front end of the nozzle 2 with respect to the die 1 is ensured at the diameter D of the front part 21 of the nozzle. In the configuration shown in FIG. 1A, the nozzle is secured to the manifold 3 via its rear end 22 by means of a screw V.
During variations in temperature of the injection system, the manifold 3 expands crosswise, to the extent of the flexibility of the screws V. In addition the nozzle may bend slightly without, however, jeopardizing the seal at the diameter D. The nozzle 2 is able to expand longitudinally in direction X, sliding along the diameter D. As a result, the position of the tip 23 of the nozzle relative to the orifice 14 provided in the die may vary depending on the expansion of the nozzle.
In particular, for small injection gates (an injection gate being the passage through which the thermoplastic material moves from the nozzle into the mould cavity), clogging of the injection gate may occur or it may lose equilibrium. Yet, if the material is heated more to make it more fluid, the nozzle expands accordingly and tends even further to shut off the injection gate. Said system is therefore particularly difficult to adjust.
In addition, in multi-cavity systems, the nozzles may expand differently leading to largely heterogeneous filling of the cavities. These problems also occur with the system in FIG. 1B, in which the manifold 3 is secured to the die 1 by means of screws V (of which only one is illustrated here) and with the system in FIG. 1C in which an elastic washer R urges the rear end 22 of the nozzle towards the manifold 3. In all these cases, the size of the injection gate is therefore dependent upon the temperature of the system.
Another system is described in document WO 2007/051857. In this document, the nozzle bears flatly upon an incompressible ring. To compensate for heat expansions of the nozzle, the rear of the nozzle is mobile by sliding inside the manifold. An elastic disc urges the front part of the nozzle onto the incompressible ring.
However, on account of the strong variations in temperature of the manifold, expansion of the manifold is also observed in a transverse direction i.e. perpendicular to the axis of the nozzle. By dilating transversally, the manifold drives with it the rear of the nozzle which is housed therein, causing bending of the nozzle. Yet, this bending, especially if the nozzle is short, causes its front face to lose some of its flat bearing, and hence a sealing loss. Flat surface sealing is much more sensitive than sealing on the diameter. Also, the elastic disc creates a heat bridge which locally cools the material circulating in the nozzle. In addition, the incompressible ring is subjected to shear forces which are detrimental thereto.
It is therefore sought to design means for ensuring a seal at the two ends of the nozzle whilst avoiding the above-mentioned problems. In particular, the injection system must allow precise, repeatable positioning of the nozzle tip to be obtained together with tolerance of the thermal expansion of the different components of the injection system, in particular the transverse expansion of the manifold.